Hi and welcome to the Allergy side of HW :)
Zyrtec can definately hide some of the allergic manifestations of a soy allergy such as hives/rashes, stuffy nose - typical allergy symtpoms. The best thing you can do to rule out a soy allergy is to be seen by a specialist. Soy allergies also can manifest them selves in gastrointestinal disturbances such as diarrhea, abdominal pain and vomiting. Soy is found in many food products so if you want to try and eliminate this from your diet, this website should prove helpful in finding what contains soy:
http://www.foodallergyinitiative.org/section_home.cfm?section_id=3&sub_section_id=7